Short carbon fiber reinforced thermoplastics (hereinafter referred to as CFRTP) containing a thermo-plastic resin as the matrix resin can be made with high productivity because of their good injection moldability, and are excellent in mechanical properties, friction and wear properties, electrical properties, dimensional stability, etc., as compared with those of a non-reinforced thermoplastic resin or a short glass fiber reinforced thermoplastic resin, so that the demand for CFRTP is rapidly increasing in use as a high performance engineering material.
CFRTP is conventionally prepared by melt blending a carbon fiber filament bundle, (namely so-called carbon fiber chopped strands) cut into a size of 3 to 10 mm and bundled with a sizing agent, or using a milled carbon fiber crushed (cut) to a size of less than 1 mm, together with, for example, thermoplastic resin pellets or powder by means of an extruder to pelletize the mixture. The pellets are molded into a molded article by an injection molding machine or an extrusion molding machine. Such methods are disclosed, for example, in British Patent 1,227,756.
The carbon fiber chopped strands and the thermoplastic resin are usually fed to the extruder by either one of the two method below:
(1) Dry blending the carbon fiber chopped strands and the thermoplastic resin, and then feeding the mixture to an extruder (a dry blending method); and
(2) First feeding the thermoplastic resin to an extruder, and then feeding the carbon fiber chopped strands into the molten thermoplastic resin (a sidefeeding method).
As is widely known, the characteristics of the CFRTP depend on the fiber length of the carbon fiber. CFRTP prepared by the use of milled fiber of an extremely short fiber length has inferior characteristics in comparison with one prepared by using of carbon fiber chopped strands because of the short length of the milled fiber. Accordingly, carbon fiber chopped strands are generally used for this purpose.
If the bundling (or the bulk density) of the carbon fiber chopped strands is insufficient, the strands cannot be fed in a stable manner to an extruder.
In the dry blending method, insufficient bundling of the carbon fiber chopped strands is liable to cause spreading of the bundle into a fiber ball like state before the compounding due to the friction with the resin and other materials, which causes the fiber to float up in the hopper of the extruder or the injection molding machine and prevents quantitative feed of the fiber. As a result, steady production of an uniform resin composition is hardly achieved, and production efficiency is lowered.
In the side feed method, insufficient bundling of the carbon fiber chopped strands also hinders the quantitative feed thereof, and in some cases it may make feed of the carbon fiber chopped strands impossible.
From these fact, one can see that the carbon fiber chopped strands are required to be well bundled.
To meet the above requirement, carbon fiber is covered (or sized) with a synthetic resin which is the same as or different from the matrix resin used in the composite.
While a large amount of sizing agent is preferably added to carbon fiber chopped strands to improve the degree of the bundling, a large amount of sizing agent lowers the dispersibility of the carbon fibers at melt blending or compounding with a thermoplastic resin, preventing the separation of the carbon fibers into single filaments and keeping the bundled state of the fibers in a molded article. Thus, the poorly dispersed bundle leads to stress concentration which lowers mechanical properties (in particular, the mechanical strength). This phenomena is remarkable in polyamides, especially nylon 66, which have a low melt viscosity.
In order to improve the bundling property of the carbon fibers, crosslinking of a sizing agent has been proposed (see, for example, JP-A-63-35308 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")). In this method, the degree of crosslinking has to be controlled in order to improve the bundling property and disperse the carbon fibers into single filaments at melt blending or compounding with a thermoplastic resin, and the production process conditions have to be strictly controlled. Thus, this process is not suitable for commercial production.
For improving the dispersibility in the resin of the carbon fiber chopped strands having a high bundling property, an increase in the blending efficiency of an extruder can be attempted by changing the screw design or other means, but this gives rise to breakage of the carbon fibers which adversely affects the improvement in various characteristics of CFRTP.
An excessive amount of sizing agent (having a decomposition point lower than the molding temperature) in the carbon fiber chopped strand results in the disadvantage that the sizing agent may be thermally decomposed to evolve gas in the case of that the chopped strand is employed with a heat resistant thermoplastic resin at a high processing temperature. This leads to a poor external appearance, lowered weld strength and other disadvantages in the molded articles produced. On the other hand, although dispersibility can be improved by decreasing the amount of sizing agent, the bundling property of the carbon fiber chopped strands cannot easily be improved in such case.
As described above, an improvement in the bundling property (or improvement in processability) is inconsistent with an improvement in dispersibility, and both improvements cannot easily be achieved simultaneously.